Final answer:
Mendel verified his principle of segregation through test crosses, which involved crossing an organism expressing a dominant trait with an organism homozygous recessive for the same trait.
Step-by-step explanation:
Mendel verified his principle of segregation through test crosses. In a test cross, an organism that expresses a dominant trait is crossed with an organism that is homozygous recessive for the same trait. If the dominant-expressing organism is a homozygote, all F₁ offspring will be heterozygotes expressing the dominant trait. If the dominant-expressing organism is a heterozygote, the F₁ offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes. The test cross further supports Mendel's postulate that pairs of unit factors (genes) segregate equally.